This invention relates generally to a hypodermic syringe for obtaining a sample of a body fluid, more particularly for drawing samples from the patient's artery or vein for blood gas analysis or other testing, and for administering fluids, more particularly for administering epidural anaesthesia.
Various syringes and methods for taking blood samples from patients are known. Such samples are normally taken by means of a syringe which includes a generally cylindrical syringe barrel having a plunger therein which, when pulled axially by an operator, creates a suction force drawing blood into the barrel through a hypodermic needle. Many tests are performed on the blood which is thus obtained from the vein of the patient. However, an increasingly important method of determining the medical status of a patient is the obtaining of arterial blood samples for testing the blood for its content of various gases. Such samples are tested for the partial pressure of oxygen, the partial pressure of carbon dioxide, the pH of the blood, the electrolyte balance, and various other tests known in the art.
Syringes previously used in obtaining arterial blood samples have generally been glass syringes, in which the cylindrical barrel is made of glass and the plunger is a ground glass rod which closely fits within the cylinder. Generally the technique for taking samples with such devices comprises, as a first step, the drawing of an anticoagulant solution, such as sodium heparin, into the syringe to replace the air in the syringe. This solution also acts as a lubricant for the walls so that the glass plunger may move relatively freely within the cylinder. The syringe is inverted and all air is expelled from the barrel and needle, along with the bulk of the anticoagulant solution, which is normally far in excess of the amount needed for the blood sample. It is extremely important that all air be expelled from the syringe, since one of the tests performed is the measurement of the amount of oxygen present in the blood, and even minute contamination with air will prevent accurate measurement of that amount. After suitable preparation of the patient, the hypodermic needle is inserted into the artery and blood is either forced into the syringe by the pressure of the blood in the artery or is drawn into the syringe barrel by withdrawing the plunger. One advantage of the glass syringe previously used is the ease with which the plunger may be moved within the lubricated barrel. The glass plunger is ground to very close tolerances, so that it is sufficiently close to the syringe barrel wall to prevent leakage but sufficiently far away to allow formation of a thin film of the anticoagulant. Even very low arterial blood pressures are usually sufficient to enter the syringe and force the glass plunger backwards without any aid from the person taking the sample. Upon entry into the syringe the blood mixes with whatever anticoagulant solution remains in the needle, syringe tip and syringe barrel after the excess has been expelled.
Glass syringes are also conventionally used in epidural anaesthesia employing the loss of resistance technique because of the low resistance to movement of the plunger in the barrel. Such technique involves loading the syringe with 2-3 ml of normal saline, sterile distilled water or air. The needle (usually 16-18 g) is then applied to the syringe. The needle is then inserted into the back towards the spinal area in question, all the while exerting gentle pressure on the plunger. When the needle tip goes through the ligamenta flava into the potential epidural space, the loss of resistance will be immediately felt by the thumb because the fluid or air will be pushed into the space. Thereafter the syringe is removed from the inserted needle and is replaced on the needle by a syringe loaded with anaethesia. This "loss of resistance" technique requires a syringe with an exceedingly smooth action and low resistance.
The glass syringes previously used have suffered from a number of disadvantages. They are expensive since the grinding requires close tolerances, in the order of 0.0007 inches clearance between the piston and the cylindrical syringe body. They are easily breakable, which is especially costly after the sample has been taken. The glass plunger and the glass barrel of each syringe must commonly be matched during the grinding by the manufacturer, since variations in grinding from one plunger to another may be sufficient to permit leakage of air or other material around the plunger, which will contaminate the sample. Thus the barrels and plungers cannot easily be individually mass produced since the plungers often cannot be satisfactorily interchanged one with another in any given barrel, as pointed out in U.S. Pat. No. 2,419,201 to Hinds. Further, because of the easy movement of the glass plunger in the cylinder, the plunger falls out of the barrel of its own weight, and normally breaks on the floor, unless the syringe is carried needle end down. Special metal holders for the glass barrel have been used to prevent this problem.
Attempts have been made to avoid these disadvantages by either manufacturing both the barrel and the plunger out of materials other than glass, such as plastics, or by using glass barrels with plastic plungers. In order to prevent leakage around the plunger, these syringes depend upon the use of a compressible and elastomeric tip at the end of the plunger, which tip generally has one or more ribs which are slightly larger in diameter than the inside of the barrel in their uncompressed state and which, when placed within the barrel, are deformed and compressed against the interior wall of the barrel and thereby form a seal. This type of seal, however, has made the movement of the plunger within the barrel difficult, thus normally requiring manual withdrawal of the plunger to obtain the blood sample, particularly when the patient's arterial pressure is low as is often the case. The handling of the syringe which is involved when manual withdrawal of the plunger is required may cause traumatization or collapse of the artery from which the blood is being taken. A further major problem has been the fact that when an axial force is applied to the plunger to expel the air and excess anticoagulant solution, the compressible elastomeric tip at the end of the plunger compresses and deforms against the floor of the syringe barrel. The initial contact of the compressible and elastomeric tip with the barrel floor is at an area spaced from the central needle opening in the floor. As force continues to be applied after initial contact the tip is compressed and deformed against the floor to move the contact area radially inwardly toward the needle opening. When the plunger is released prior to the insertion of the hypodermic needle into the artery, the pressure on the compressible and elastomeric tip is also released whereby it recovers its normal shape and in so doing moves back slightly from the barrel floor until its only contact with such floor is its initial contact area thereby drawing a small amount of air into the tip of the hypodermic needle which becomes mixed with the blood sample thereby increasing its oxygen and nitrogen content. Also, air bubbles tend to collect on the forward face of the elastomeric tip particularly at the area immediately adjacent the point of contact with the syringe barrel wall. Since the samples which are drawn to test for the amount of oxygen and carbon dioxide in the blood are very small, e.g., 2, 5 or 10 ml, even minute amounts of oxygen and carbon dioxide leaked into the sample or in the form of air bubbles have potentially adverse effects on the results obtained. The compressibility of the plunger tip also causes non-uniformity in the amount of anticoagulant left in the syringe barrel, syringe tip and hypodermic needle. As can be readily appreciated, the amount left will depend upon the amount of pressure used to expel the air and excess anticoagulant since greater pressure will compress and distort the compressible plunger tip to a greater degree, thus expelling more anticoagulant. If too little anticoagulant solution remains to be mixed with the blood, the blood may coagulate prior to testing and thus adversely affect the results obtained. If, on the other hand, too much anticoagulant solution is left in the syringe, its presence may adversely affect the test, as is known in the art.